Modern aircraft are normally operated to accommodate passengers, cargo, or a combination of the two. In either case, the goal is to generate maximum revenues for each flight. In the case where the aircraft is fitted for passengers, this may be accomplished by adding extra seats. For cargo transportation, clearly the goal is to accommodate the largest volume of parcels and freight possible given the total volume available in the aircraft fuselage.
Most commonly, cargo is loaded into a freighter aircraft either in cargo containers or secured to cargo pallets. To accommodate and secure these containers or pallets in flight, freighter air craft are fitted with ball mats, floor rollers, and floor restraints occasionally referred to as "locks." The locks are secured to seat tracks at specific locations on the cargo floor. The seat tracks are essentially grooves to which passenger seats are typically secured. The locks are positioned on the cargo floor so as to provide fore, aft and vertical restraint for the cargo containers or pallets.
The locks or restraints which are currently used have retractable spring-loaded bear trap-type pawls which, when erected, engage the edges of the containers or pallets. When retracted, the restraints lie below the transport plane. The transport plane is defined as a horizontal plane coincident with the lower surface of the container or pallet when in full contact with the cargo rollers or ball units. For loading and unloading purposes, retracted restraints must lie below the transport plane to permit free movement of the Unit Load Devices ("ULD's", i.e., cargo containers and pallets) within the fuselage main deck cargo area.
Typically, when loading or unloading a freighter aircraft, the first or most forward ULD is initially moved forward (or toward the front of the aircraft) of the in-flight position to fixed stops called rollout stops. The first ULD is moved to the rollout stops to provide sufficient room for personnel to erect or retract the restraint pawls.
In recent years, a modification was made by operators to the cargo systems of many narrow-body freighter aircraft, such as the Boeing-727 and the Douglas DC-8. This modification added a half size ULD in front of the normal main deck ULD complement. Some operators termed this as the "ICU" modification which stands for "Increased Cube Utilization" (i.e., more cubic feet available). This modification was desirable to increase revenue payloads because for those flights with lower density cargo, the aircraft might become volume limited ("cubed out") before it approached the established maximum gross weight limit for operation ("grossed out").
While the payload volume was increased by this modification, an unfortunate side effect occurred which generated loading and unloading difficulties. It was no longer possible to move the first ULD forward toward the original rollout stops to generate gaps between the ULDs. The additional half size ULD used up the space which was previously available. That space had been useful to permit separating the other ULD's sufficiently to erect and retract their mutually shared floor restraints. But after the addition of the ICU module, the new configuration required that the restraint pawls be erected to engage each ULD as it was loaded into position. As always, the last ULD loaded (and the first to be unloaded) had to be positioned on the ballmat in the doorway area adjacent to the main cargo door. However, now in addition, the fore and aft floor restraint pawls had to be erected before loading this final ULD into position. Otherwise, the final ULD would sit on top of the cargo restraint pawls before they had been erected an locking would be impossible. Threading this ULD into position under the erected restraint pawls and unloading the ULD from this position has proven to be a frequently difficult and time-consuming task and is the reason for the development of the cargo hardware described herein.
The problem which has developed concerns the cargo position commonly referred to as 3-Left or 3L position. As mentioned, the 3L cargo position is immediately adjacent to the main cargo door on the upper deck, and it is the last position in which cargo is secured prior to flight. It is also the first position unloaded from the aircraft. The difficulty with the 3L position arises because the 3L position must be loaded with the conventional restraint pawls erected. The reason that the pawls are erected is that they must hold the cargo containers in place on either side, fore and aft, of the 3L position.
In order to place the cargo module in or remove the container from the 3L position, the cargo container must be "threaded" under the erected pawls from a mechanical loader or cargo handler from outside the cargo hold. Since the aircraft sits on landing gear with compressible air-oil shock struts, as the aircraft is loaded and unloaded, the center of gravity shifts and both the deck height and angle may change. Mechanical loaders are capable of compensating for height changes, but few can accommodate deck angle changes. Once a deck angle difference has been created between the aircraft and the loader, the container impacts the erected pawls and may become wedged in place. This problem is especially acute when the cargo container is half in the aircraft and half on the loader. The end result is that it can take as long to load or unload the 3L position as it does to load or unload the remainder of the aircraft.